1. Field of the Invention
Embodiments of the present disclosure are directed in to yellow-green to yellow-emitting phosphors based on aluminates that contain the rare earth terbium (Tb). Such phosphors are applicable to a number of different technologic areas, including general lighting systems, white light illumination systems based on white LEDs, signal lights, indicator lights, etc., as well as display applications such as display backlighting, plasma display panels, LED-based display panels, and the like.
2. Description of the Related Art
Embodiments of the present invention are directed to aluminate-based phosphors that, when activated by cerium, and when doped with the rare earth terbium (Tb) emit visible light in the yellow-green to yellow portion of the electromagnetic spectrum. The phosphor may also include the rare earths lutetium (Lu) and/or gadolinium (Gd). The phrase “visible light in the yellow-green to yellow portion of the electromagnetic spectrum” is defined to mean light having a peak emission wavelength of about 550 nm to about 600 nm. Such phosphors may be used in commercial markets where white light is generated using so-called “white light LEDs,” noting as an aside that this term is somewhat of a misnomer, since light emitting diodes emit light of a specific monochromatic color and not a combination of wavelengths perceived as white by the human eye. The term is nonetheless entrenched in the lexicon of the lighting industry.
Historically, YAG:Ce (yttrium aluminate garnet activated with cerium) has been used to supply the yellow component of the light in the lighting systems mentioned above. In comparison to other phosphor hosts, particularly those based on the silicates, sulphates, nitridosilicates, and oxo-nitridosilicates, YAG:Ce has a relatively high absorption efficiency when excited by blue light, is stable in high temperature and humidity environments, and has a high quantum efficiency (QE>95%), all the while displaying a broad emission spectrum.
One disadvantage to using a YAG:Ce based phosphor, other than inadequate color rendering in some situations, is that the peak emission of this phosphor is too long, that is to say, too deep towards the orange or red for use as a luminescent source in, for example, a backlighting application. An alternative to YAG:Ce is the cerium doped Lu3Al5O12 compound (LAG:Ce), which has the same crystalline structure as YAG:Ce, a similar temperature and humidity stability as the yttrium-based compound, and likewise quantum efficiency. Despite these similarities, LAG:Ce exhibits a different peak emission wavelength than its YAG counterpart; in the lutetium case, this peak wavelength is at about 540 nm. This emission wavelength is still not short enough, however, to be ideal for certain applications such as backlighting applications, and general lighting applications, where appropriate.
Thus, what is needed in the art, particularly in fields related to backlighting technologies and general lighting, is a phosphor with a structure comparable to a garnet in terms of temperature and humidity stability, but having at the same time a peak emission wavelength ranging from about 550 nm to about 600 nm. According to the present embodiments, these challenges may be addressed by providing lutetium (Lu) aluminate-based phosphors that include the rare earth terbium (Tb). The phosphors may also include the rare earth gadolinium (Gd).